Dave S wrote

The aircraft doesnt distinguish between a forward and a side slip, so
why should the CFI?

Probably because the FAA makes the distinction in AC 61-21A Flight
Training Handbook (pages 102-103). I don't have the replacement
for that AC, but it's probably in there also. I can assure you that
an Examiner will expect a CFI to have instructed the student on the
difference.

Bob Moore